FIG. 1(A) shows a known oscillator. The oscillator includes a dielectric resonator 104 provided on a surface of a dielectric substrate. A signal line and a main line 103 coupled to the resonator are provided on a mount board on which the dielectric resonator 104 is disposed. An electrode gap for DC-cutting (hereinafter, this structure is referred to as a DC-cut structure) 106 is provided in the main line 103. Thus, the main line 103 includes two components 103A and 103B. A negative resistance element 101 is connected to the main-line component 103A, and a termination resistor 102 is connected to the main-line component 103B. Accordingly, a band-reflection-type oscillator is configured (for example, see Patent Document 1).
In addition to the dielectric resonator described above, a waveguide dielectric resonator 110 including a dielectric block 111 having a substantially rectangular parallelepiped shape as shown in FIG. 1(B) is known. A configuration in which openings 113A and 113B are arranged in an outer-surface electrode provided on an outer surface of the dielectric block 111 and in which line patterns (input/output electrodes) 114A and 114B each having one end that is short-circuited with respect to the outer-surface electrode 112 and the other end that is open with respect to the outer-surface electrode 112 are provided in the openings 113A and 113B, respectively, is publicly known (for example, see Patent Document 2).
For each of the resonators described above, an input/output electrode of a dielectric substrate, a dielectric block, or the like (hereinafter, simply referred to as a dielectric member) is connected to a signal line of a mount board and part of an outer-surface electrode of the dielectric member is connected to a ground electrode. Accordingly, an equivalent short-circuited point is provided in the input/output electrode, and the dielectric resonator can thus be excited.
It is known that, in a case where a DC-cut structure is implemented with the provision of an electrode gap in a microstrip line, such as a main line, when each of the main-direction-of-line lengths of portions of lines that face each other across the electrode gap is set to one quarter the wavelength of a passing high-frequency signal, minimum signal loss can be achieved (for example, see Non-Patent Document 1).    Patent Document 1: Japanese Examined Utility Model Registration Application Publication No. 6-48974    Patent Document 2: Pamphlet of International Publication No. WO2002/078119    Non-Patent Document 1: Yoshihiro KONISHI, “Maikuroha Kairo no Kiso to Sono Ouyou (Basics and Applications of Microwave Circuits), p. 318
In the above-described known technology described in Patent Document 1, the signal line and the main line disposed on the mount board are disposed on a portion of the substrate outside the resonator. Thus, the size of the mount board on which the dielectric resonator is disposed tends to be increased, thus increasing the size of the entire device.
In addition, in the known technology described in Patent Document 2, even if the relative positions of an outer-surface electrode and an input/output electrode of a dielectric member are only slightly shifted, the amount of coupling between the dielectric resonator and the input/output electrode is greatly changed. As a result, the amount of coupling with an input/output electrode is likely to vary according to the dielectric resonator used. Thus, in order to reduce the variation in the electric characteristics of a resonator, it is necessary to accurately perform fine adjustment of the outer shape of an input/output electrode after the input/output electrode is formed.
In addition, for such a dielectric resonator, the phase of a reflection signal or a transmission signal from the resonator is fixed in accordance with the distance from a total reflection end (short-circuited point) of the input/output electrode. Thus, fixed constraints may be imposed on the positions and sizes of other circuit elements and lines on the connected mount board. Accordingly, circuit design of the mount board is strictly constrained.
In the known technology described in Non-Patent Document 1, when, with respect to the wavelength of a high-frequency signal propagating in the signal line, the length of comb-like portions of the DC-cut structure is set to one quarter the wavelength of the high-frequency signal, the transmission loss of the signal can be reduced to the minimum. However, if an electrode gap is set to a different length, the transmission loss of the signal significantly increases. Thus, it is necessary to accurately adjust the shape of the power-feeding line.